Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors.
OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.
Organic electroluminescent devices utilize the radiative decay of excitons formed inside the emissive layer. The position of exciton formation and migration play very important role on the stability and efficiency of the devices. When holes and electrons are injected to the devices, they travel in the emissive layer, recombine, and form excitons. When the recombination zone is too narrow or close to HTL and ETL interfaces, a large buildup of charge and high concentration of excitons can occur, which can cause polaron-exciton interaction and triplet-triplet annihilation. These interactions can adversely affect the device performance, generally shortening the device lifetime. In order to increase device efficiency and improve lifetime, it is desirable to have a wider recombination zone and lower exciton concentration in the emissive layer. Therefore, the charge transporting properties of the emissive layer is important.
A driving force behind the use of organic electroluminescence in displays and lighting has been the introduction of red and green electrophosphorescent devices with up to 100% internal quantum efficiency. However, achieving deep blue electrophosphorescence with both high efficiency and long-term operational stability remains a challenge. The design of robust and efficient blue phosphors free of electrochemically reactive moieties offers one possible solution. For example, Sajoto, T. et al. (Blue and Near-UV Phosphorescence from Iridium Complexes with Cyclometalated Pyrazolyl or N-Heterocyclic Carbene Ligands. Inorg. Chem. 44, 7992-8003 (2005)) demonstrated tris-cyclometalated iridium (III) complexes based on the thermodynamically stable N-heterocyclic carbene (NHC) ligands, or Ir(C^C:)3. Compared with blue Ir complexes using fluorination to obtain a wide energy gap, such as bis[2-(4,6-difluorophenyl)pyridinato-C,N](picolinato) iridium (III) (FIrpic) and bis[2,4-difluorophenylpyridinato]tetrakis(1-pyrazolyl) borate iridium (III) (FIr6), the NHC approach provides more saturated blue emission.
However, previously reported deep blue PHOLEDs using either NHC- or other ligand-based metal complexes that can accommodate highly energetic excited states are subject to a pronounced external quantum efficiency (EQE) roll-off at a high brightness; i.e., the current densities at the half maximum EQE, J1/2, are typically <50 mA/cm2 due to exciton and electron leakage from the PHOLED emissive layer (EML), as well as strong bimolecular annihilation. Inventors have discovered that when the same compounds are used for both the emitter dopant in the EML and for the neat electron/exciton blocking layer (EBL) film next to the EML, significant improvement in device efficiency can be achieved.